The invention relates to a method for recovering and/or recycling lumpy silicon starting material by crushing the starting material, whereby the recovered or recycled material is melted, and from this melt, crystals are grown as silicon block, tube, or strip, for example.
To produce silicon crystals, large waste pieces are normally used, such as fragments or chunks left over from poly-Si production, sawed-off end pieces of Czochralski or float-zone crystals, edge pieces from discarded blocks, and spherical or splinter-shaped broken material.
With Si-block casting technique, fine Si-granulate is required in order to fill gaps and hollow spaces during the filling of crucibles, in order to achieve high filling rates.
With the Czochralski (CZ) production technique, Si-chunks (large pieces with a diameter of several centimeters) are used for charging. For post-charging, some spherical material with a particle size of 0.2-5 mm is also used. Such particles are normally produced using a fluidized bed separation method. With this material, which is approximately one millimeter in size, considerably finer gradation is achieved in charging.
Also with the edge-defined film-fed growth (EFG) method, in which Si-wafers in the form of polygonal tubes are drawn directly out of the melt, a granulate with a particle-size in the millimeter range is used, because due to the crucible volume, finer charging is required. In the state of the art, spherical material is currently used exclusively.
In addition to this spherical material, there are also irregularly shaped particles of the corresponding size that can be produced by breaking them off solid polycrystalline silicon. This material shows a percentage of approximately 0.1% to 10% (mean value 1%) due to the production method (pyrolytic growth from the gas phase on a rod), a distinctly elongate structure with a high aspect ratio (length to smallest upward measurement; hereinafter the simplified ratio of length to width will be used). This substance is recognizable from the material containing needles with an aspect ratio (length:width) that is much higher than 1, as well as particles with an aspect ratio of 10-30. These can only be filtered out only with difficulty. But the elongate structure is disadvantageous when conveying through pipes with a small diameter or those that have curves and corners.
From WO-A-2005/088680, a method is known for conveying solids particles of irregular geometry that have a large aspect ratio and can be conveyed through pipes by first adding solid particles with a regular geometrical form. In order to ensure unobstructed conveyance, the solids particles with an irregular geometrical form should have a aspect ratio of ≦3.
Recycled Si-block remainders, chunks, and sections of Cz crystals, due to their size, create hollow spaces in the bulk material, so that the bulk density is relatively low. These hollow spaces are therefore filled with finer, and as stated above, often spherical material.
U.S. Pat. No. 4,084,024 describes a method of producing high purity silicon. The silicon material produced displays a uniform geometry. In order to produce single crystal silicon blocks with the required doping, according to WO-A-86/06764, mixtures of different silicon starting materials are used.
According to U.S. Pat. No. 5,304,241, a powder mixture consisting of silicon dioxide is used as a filler for a sealing agent for electronic components. In order to obtain the pulverized material, silicon dioxide particles are crushed using a pulverizer, which is lined with a ceramic layer in order to avoid contamination. The aspect ratio of the pulverized particles ranges from 1.2 to 1.4.